Seagate Portable HD won't work with Monterey?

Make sure it is formatted in a mac OS compatible file system

File system formats available in Disk Utility on Mac - Apple Support

Erase and reformat a storage device in Disk Utility on Mac - Apple Support

It is possible it is formated as NTFS and you previusly used some third party software to be able to write to it.

If this is not the case. It could still be worth it to reformat it. But in any case make sure you move any content you want to keep out of the disk using another computer first.

You are not the only one having this problem; that happened to me after upgrading to Monterey, so I went back to Catalina and am still having the same problem. Never ever had an issue using Mojave. Others have written about as well. I watched a good Youtube last night, followed the instructions, and the Seagate seems to be working better now (fingers crossed). Had to go into Recovery to be able to erase, format in APFS. Now there are containers that are added in Catalina and Monterey. https://www.youtube.com/watch?v=caYfZWyN4CQ

Use Disk Utility to format:

Intro to Disk Utility on Mac - Apple Support

File system formats available in Disk Utility on Mac - Apple Support

What format you use depends upon its use.

macOS Monterey.

Yes, what I said was based on that. It is possible it is formatted in a way that does not have native support in mac OS.

And that the software used to make it compatible does not work on Monterey

How do I reformat my Seagate hard drive that is not working with Monterey